An infrared radial GRIN lens based on chalcogenide glass and a preparation method thereof

Abstract

This invention discloses an infrared radial GRIN lens based on chalcogenide glass and its fabrication method. The matrix material of the lens is a chalcogenide glass with a molar composition of 55GeS2-20In2S3-25NaI. The lens has a radially distributed potassium ion concentration gradient inside, which continuously decreases from the radial outer surface towards the center, resulting in a radially negative gradient distribution of refractive index that continuously increases from the radial outer surface towards the center. This lens possesses a wide infrared transmission window and a high refractive index, with a large refractive index control range and low cost. Its maximum refractive index difference Δ at a wavelength of 10.6 µm is [not specified in the original text]. n The gradient depth is approximately 1 mm, with a value of approximately 0.015, and the distribution is highly consistent in all directions. This invention provides a key optical element for realizing a high-performance, miniaturized infrared imaging system. It is fabricated using an ion exchange method, a simple and controllable process that optimizes raw material costs.

Description

Technical Field

[0001] This invention relates to the field of infrared optical materials technology, specifically to an infrared radial GRIN lens based on chalcogenide glass and its fabrication method. Background Technology

[0002] In complex optical systems, especially in multi-path imaging and fusion devices, aberrations are a core issue limiting image quality. Traditional correction methods typically rely on aspherical lenses or complex lens combinations, which significantly increases system cost and assembly complexity, and also limits the development of lightweight and integrated systems. The emergence of gradient refractive index (GRIN) materials offers a new approach to solving this problem. The main characteristic of GRIN materials is that their refractive index can continuously change with spatial position. By precisely designing the refractive index gradient distribution, light can be locally controlled, thereby effectively correcting various aberrations such as spherical aberration and astigmatism. It is particularly noteworthy that radially gradient refractive index plane lenses achieve aberration correction effects similar to aspherical lenses, but with significantly reduced fabrication complexity and cost. Furthermore, GRIN optical elements typically feature large numerical apertures and short focal lengths, which are beneficial for improving light collection efficiency and system compactness, making them highly suitable for the design of high-performance, miniaturized optical systems. Despite significant progress in aberration theory, ray tracing, and lens design, the fabrication of infrared GRIN lenses still faces numerous challenges, primarily due to the higher requirements placed on materials and processes by the unique physical properties of infrared light.

[0003] Among infrared materials, chalcogenide glasses have become ideal candidate materials for fabricating infrared GRIN lenses due to their excellent comprehensive properties. Chalcogenide glasses are mainly composed of sulfur, selenium, and tellurium, and often incorporate elements such as germanium, arsenic, and antimony to adjust their properties. They possess a wide infrared transmission window (typically covering 0.5–20 μm), good glass-forming ability and chemical stability, optical properties that can be continuously controlled through composition, and a low temperature coefficient of refractive index, which helps suppress thermal defocusing and improve system temperature stability. Compared to traditional infrared materials such as single-crystal germanium, chalcogenide glasses are lower in cost and more suitable for large-scale production, thus being regarded as one of the key materials driving the development of high-performance, miniaturized infrared imaging systems.

[0004] In recent years, modifying chalcogenide glasses by introducing alkali metal halides (such as CsCl and CsI) has become an important strategy for optimizing their performance. This provides a structural basis for subsequent refractive index control and the preparation of gradient refractive index materials through ion exchange processes. Ion exchange is a mature glass strengthening and modification process that replaces ions in molten or solid salts with ions inside the glass, thereby creating an ion concentration gradient within the material and inducing a refractive index gradient. For example, Fourmentin et al. prepared infrared GRIN lenses in chalcogenide glasses using a molten salt bath ion exchange method, but this process is time-consuming and the nitrates used are expensive. Subsequently, the team proposed a rapid solid-solid Na... + / Ag + While ion exchange technology improves the diffusion rate and avoids molten salt corrosion, it uses expensive silver nitrate, resulting in high process costs and hindering large-scale production.

[0005] Therefore, the fabrication of infrared GRIN lenses in existing technologies still suffers from problems such as complex processes, limited refractive index control range, and high costs of key raw materials. There is an urgent need to develop a low-cost, process-controllable infrared radial GRIN lens suitable for mass production and its fabrication method. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide an infrared radial GRIN lens based on chalcogenide glass and its fabrication method, addressing the shortcomings of existing technologies. This lens possesses a wide infrared transmission window (0.5–12 µm) and a high refractive index, with a large refractive index control range and low cost. Its maximum refractive index difference Δ at a wavelength of 10.6 µm is [not specified in the original text]. n The refractive index gradient is approximately 0.015, with a gradient depth of about 1 mm and good uniformity in all directions. The built-in refractive index gradient of the lens in this invention endows it with the optical capability to correct aberrations (such as spherical aberration), providing a key optical component for realizing high-performance, miniaturized infrared imaging systems. The lens of this invention is fabricated using an ion exchange method, a simple and controllable process that optimizes raw material costs. The fabricated lens has significant application potential in miniaturized, lightweight, high-performance infrared optical systems and is suitable for pilot-scale and even large-scale production.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: an infrared radial GRIN lens based on chalcogenide glass, wherein the matrix material of the lens is chalcogenide glass with a molar composition of 55GeS2-20In2S3-25NaI, and the lens has a potassium ion concentration gradient distributed radially inside, which continuously decreases from the radial outer surface of the lens to the center, thereby resulting in a radial negative gradient distribution of refractive index that continuously increases from the radial outer surface of the lens to the center.

[0008] The lens of this invention uses a specific composition of 55GeS2-20In2S3-25NaI chalcogenide glass as the matrix material. This matrix material itself possesses a wide infrared transmission window of 0.5–12 µm and tunable optical properties. A radial potassium ion concentration gradient continuously decreasing from the outer radial surface to the center of the lens is successfully constructed within this matrix material, and K0 is utilized… + The negative correlation between concentration and refractive index directly yielded a radially increasing negative gradient distribution from the outer radial surface of the lens towards the center. Experimental measurements show that the maximum refractive index difference Δ at a wavelength of 10.6 µm is... n The refractive index gradient is approximately 0.015, with a gradient depth of about 1 mm, and good uniformity in all directions. This built-in refractive index gradient of the lens of the present invention enables it to inherently possess the optical capability to correct aberrations (such as spherical aberration), providing a key optical element for realizing high-performance, miniaturized infrared imaging systems.

[0009] A method for fabricating the above-mentioned infrared radial GRIN lens based on chalcogenide glass includes the following steps: (1) Weigh the raw material according to the molar composition of 55GeS2-20In2S3-25NaI and put it into a clean quartz tube; (2) Evacuate the quartz tube containing the raw materials to a vacuum level of 1×10⁻⁶. -4 The pressure below Pa is then melted and sealed to obtain a sealed quartz tube; (3) Place the obtained sealed quartz tube in a swing furnace and melt it at a temperature of 900-990℃ for 12-25 hours to allow the raw materials to react fully and become homogenized; (4) After melting, the glass melt is quenched to solidify into a glass ingot. Then, the glass ingot is annealed to eliminate thermal stress and obtain a uniform 55GeS2-20In2S3-25NaI chalcogenide glass block. (5) The obtained chalcogenide glass block is processed into a glass column, and together with potassium oleate powder, it is placed into another clean quartz tube and evacuated to 1×10⁻⁶. -4 Below Pa and fused to seal the quartz tube; (6) Keep the sealed quartz tube at 250-270℃ for 30-40 hours to melt the potassium oleate and allow the K in the molten potassium oleate to react. + With Na in the glass column + Ion exchange occurs, forming radial K + Concentration gradient; (7) After the ion exchange is completed, the temperature is slowly lowered to room temperature. Then the sample is taken out, cleaned and dried, and then subjected to stress relaxation heat treatment: the temperature is raised to 150-240℃ and held for 10-60 hours, and then slowly cooled to room temperature. (8) Cut off both ends of the sample along the axial direction to obtain a preform, and then grind and polish the preform to obtain the infrared radial gradient refractive index lens.

[0010] This invention provides a complete and controllable preparation process. Its core innovation lies in first preparing an optically homogeneous, stress-free chalcogenide glass matrix material of a specific composition using a vacuum melting-quenching method; then, innovatively employing potassium oleate as the salt bath medium for ion exchange, replacing the expensive nitrates (such as silver nitrate) commonly used in existing technologies. K+ ion exchange is then carried out at a relatively low temperature of 250–270°C. + / Na + Ion exchange successfully introduced an ion concentration gradient into the glass column. In the preparation method of this invention, after the ion exchange process (i.e., ion exchange heat treatment) in step (6), a staged heat treatment strategy after ion exchange is specially designed, namely, long-term stress relaxation heat treatment at 150-240℃. This step is not a simple annealing, but is designed based on the thermal diffusion kinetics of ions in the glass: below the glass transition temperature T g But at sufficiently high temperatures, K + and Na + Short-range adjustments can be made to mitigate localized stress concentrations caused by differences in ion size, preventing cracking or delamination of the sample during cooling or processing. This step effectively promotes further uniform ion diffusion and releases interfacial stress generated by ion exchange, significantly improving the mechanical integrity of the material and the yield of the finished product, thus preventing cracking of the sample during cooling or subsequent processing. The entire preparation process is carried out under vacuum, isolating it from interference from oxygen and moisture, resulting in good repeatability. Furthermore, the overall cost is significantly lower than processes using precious metal salts such as silver nitrate, making it more suitable for large-scale production.

[0011] Preferably, in step (1), the raw materials include the corresponding compounds or elemental raw materials used to form GeS2, In2S3 and NaI.

[0012] As a further preferred option, in step (1), the raw materials are germanium, indium, sulfur, and sodium iodide, wherein the purity of germanium, indium, and sulfur is 5N, and the purity of sodium iodide is 2N. Using raw materials with the above-mentioned purity can effectively reduce the introduction of impurity elements, ensure that the chalcogenide glass matrix material has high intrinsic transmittance in a wide infrared band (especially in the mid-to-long wavelength range), and avoid the influence of impurity absorption peaks on optical performance.

[0013] Preferably, in step (6), the sealed quartz tube is slowly heated to 250–270°C over 2–5 hours and held at that temperature for 30–40 hours. This slow heating process helps ensure uniform heating of the potassium oleate powder and glass column inside the quartz tube, avoids cracking of the glass or the encapsulated quartz tube due to thermal shock, and ensures stable melting of the potassium oleate. Sufficient holding time ensures that K… + Sufficient time for Na in the glass +Sufficient diffusion and exchange are essential to achieving the predetermined millimeter-level diffusion depth and the required refractive index gradient amplitude Δ. n The key process control parameters improve the repeatability of the process and the consistency of gradient formation.

[0014] Preferably, in step (7), after ion exchange, the process of slowly cooling to room temperature is as follows: the temperature is slowly cooled from 250-270℃ to room temperature over 30-50 hours; the specific process of stress relaxation heat treatment is as follows: the temperature is raised from room temperature to 150-240℃ over 30-50 hours and held for 10-60 hours, followed by slow cooling to room temperature over 30-50 hours. The slow cooling for 30-50 hours after ion exchange can effectively suppress the thermal stress accumulated inside the glass due to the sudden drop in temperature and prevent the sample from cracking during the cooling stage. The stress relaxation heat treatment also adopts a slow heating and cooling rate (30-50 hours each) and is held for a long time (10-60 hours) within a specific temperature range (150-240℃). The above heating and cooling process design is based on the viscosity-temperature characteristics of chalcogenide glasses and the ion thermal diffusion kinetics, which allows the glass network structure to relax locally at a temperature below the transition temperature but high enough, so that the exchanged K + and the remaining Na + Short-range adjustments can effectively mitigate the impact of K. + with Na + Localized stress concentration caused by differences in ionic radii. This step is crucial for eliminating interfacial stress, improving the overall mechanical strength of the glass, and ensuring the feasibility and yield of subsequent precision optical processing (cutting, polishing).

[0015] Compared with the prior art, the present invention has the following advantages: 1) This invention utilizes a chalcogenide glass-based infrared radial GRIN lens with a specific composition of 55GeS2-20In2S3-25NaI chalcogenide glass as the matrix material. This not only gives the lens a wide infrared transmission window (0.5–12 µm) and a high refractive index, but more importantly, its composition is highly tunable. By adjusting the ratio of Ge, In, and S, key parameters such as refractive index and transition temperature can be continuously controlled, providing a rich material basis for gradient design. Compared to traditional infrared materials such as single-crystal germanium, this material system does not contain expensive rare elements, significantly reducing raw material costs. The maximum refractive index difference Δ at a wavelength of 10.6 µm is [not specified in the original text]. n The refractive index gradient is approximately 0.015, with a gradient depth of about 1 mm, and good uniformity in all directions. This built-in refractive index gradient of the lens of the present invention enables it to inherently possess the optical capability to correct aberrations (such as spherical aberration), providing a key optical element for realizing high-performance, miniaturized infrared imaging systems.

[0016] 2) The preparation method of this invention innovatively uses potassium oleate as the salt bath medium for ion exchange, replacing the expensive nitrates (such as silver nitrate) commonly used in existing technologies. Potassium oleate is not only cheaper and easier to store, but also has suitable viscosity and chemical stability in the molten state, which can effectively promote K+ exchange. + Na in glass + The exchange reaction is achieved while avoiding excessive corrosion of the quartz container at high temperatures. The combination of potassium oleate and 55GeS2-20In2S3-25NaI chalcogenide glass optimizes raw material costs while ensuring process feasibility. The prepared lens has significant application potential in miniaturized, lightweight, and high-performance infrared optical systems, and is suitable for pilot-scale and even large-scale production.

[0017] 3) The preparation method of this invention also innovatively designs a staged heat treatment strategy after ion exchange, namely, long-term stress relaxation heat treatment at 150-240℃. This step can effectively promote further uniform diffusion of ions and release the interfacial stress generated by ion exchange, significantly improving the mechanical integrity of the material and the yield of the finished product, and avoiding cracking of the sample during cooling or subsequent processing. In addition, the entire preparation process is carried out under vacuum, isolating the interference of oxygen and moisture, resulting in good repeatability, and the overall cost is significantly lower than that of processes using precious metal salts such as silver nitrate, making it more suitable for large-scale production. Attached Figure Description

[0018] Figure 1 A physical image of the lens obtained in Example 1 (placed on a white paper printed with the word "NBU"). Figure 2 The image shows the three-dimensional refractive index distribution of the lens prepared in Example 1 obtained under an interferometer. Figure 3 The image shows the refractive index distribution at a wavelength of 10.6 µm, obtained by taking the refractive index profiles along the X and Y axes on the lens prepared in Example 1. Detailed Implementation

[0019] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0020] Example 1: An infrared radial GRIN lens based on chalcogenide glass. The matrix material of the lens is a chalcogenide glass with a molar composition of 55GeS2-20In2S3-25NaI. The lens has a radially distributed potassium ion concentration gradient inside, which continuously decreases from the radial outer surface of the lens towards the center, resulting in a radially negative gradient distribution of refractive index that continuously increases from the radial outer surface of the lens towards the center. The preparation method of this lens includes the following steps: (1) Weigh germanium, indium, sulfur raw materials with a purity of 5N and sodium iodide raw material with a purity of 2N according to the molar composition of 55GeS2-20In2S3-25NaI, and put them into a clean quartz tube with a wall thickness of 2 mm, an inner diameter of 8 mm and a length of 280 mm, which has been soaked in aqua regia, washed with distilled water and dried. (2) Connect the quartz tube containing the raw material to the molecular pump system and evacuate to 1×10⁻⁶. -4 A sealed quartz tube with a length of approximately 200 mm was obtained by using an oxyhydrogen flame for sealing below Pa. (3) The obtained sealed quartz tube is placed in a programmable temperature-controlled swing furnace and melted at 900-990℃ for 12-25 hours. During this process, the furnace body of the swing furnace is continuously swinging to fully melt and react the raw materials and achieve homogenization of the components. (4) After melting, the sealed quartz tube is quickly immersed in cold water for quenching, so that the glass melt solidifies into a glass ingot. Then the glass ingot is transferred to a programmed annealing furnace for annealing to eliminate thermal stress and obtain optically uniform 55GeS2-20In2S3-25NaI chalcogenide glass block that meets the requirements. (5) The obtained chalcogenide glass block is processed into a glass column of a specified size, and together with sufficient potassium oleate powder, it is placed into another clean quartz tube, and the tube is evacuated to 1×10⁻⁶. -4 Below Pa, the quartz tube is sealed with an oxyhydrogen flame to prevent potassium oleate from oxidizing at high temperatures; (6) Place the sealed quartz tube in a programmed annealing furnace, slowly raise the temperature to 250°C over 2 hours and hold for 30 hours to melt the potassium oleate and allow the K in the molten potassium oleate to react. + With Na in the glass column + Ion exchange occurs, forming radial K + Concentration gradient; (7) After the ion exchange is completed, the temperature is slowly reduced to room temperature, that is, the temperature is slowly reduced from 250℃ to room temperature within 30 hours; then the sample is taken out, cleaned and dried, and then put into the annealing furnace again for stress relaxation heat treatment. The specific process is: the temperature is raised from room temperature to 240℃ within 30 hours and kept at that temperature for 30 hours to promote further uniform diffusion of ions and release interfacial stress, and then slowly cooled to room temperature within 30 hours. (8) Cut off both ends of the sample along the axial direction to obtain a preform of a specified length. Then grind and polish the preform to obtain an infrared radial gradient refractive index lens. See the attached image for the actual object. Figure 1 .

[0021] The refractive index distribution profiles of the lenses prepared in Example 1 were measured using an MZ interferometer to obtain... Figure 2 The refractive index distribution diagram shown indicates that the test results of the fabricated lens K... +The diffusion depth is approximately 1 mm, with the maximum refractive index difference Δ at the 10.6 µm wavelength band. n It is approximately 0.015 and maintains high transmittance in a wide infrared band of 0.5–12 µm. Figure 3 The refractive index profiles obtained by taking the X-axis and Y-axis on the lens at a wavelength of 10.6 µm show that the refractive index distribution of the lens has a high degree of consistency in all directions.

Claims

1. An infrared radial GRIN lens based on chalcogenide glass, characterized in that, The matrix material of the lens is a chalcogenide glass with a molar composition of 55GeS2-20In2S3-25NaI. The lens has a radially distributed potassium ion concentration gradient inside, which continuously decreases from the radial outer surface of the lens to the center, resulting in a radial negative gradient distribution of refractive index that continuously increases from the radial outer surface of the lens to the center.

2. A method for fabricating an infrared radial GRIN lens based on chalcogenide glass as described in claim 1, characterized in that, Includes the following steps: (1) Weigh the raw material according to the molar composition of 55GeS2-20In2S3-25NaI and put it into a clean quartz tube; (2) Evacuate the quartz tube containing the raw materials to a vacuum level of 1×10⁻⁶. -4 The pressure below Pa is then melted and sealed to obtain a sealed quartz tube; (3) Place the obtained sealed quartz tube in a swing furnace and melt it at a temperature of 900-990℃ for 12-25 hours to allow the raw materials to react fully and become homogenized; (4) After melting, the glass melt is quenched to solidify into a glass ingot. Then, the glass ingot is annealed to eliminate thermal stress and obtain a uniform 55GeS2-20In2S3-25NaI chalcogenide glass block. (5) The obtained chalcogenide glass block is processed into a glass column, and together with potassium oleate powder, it is placed into another clean quartz tube and evacuated to 1×10⁻⁶. -4 Below Pa and fused to seal the quartz tube; (6) Keep the sealed quartz tube at 250-270℃ for 30-40 hours to melt the potassium oleate and allow the K in the molten potassium oleate to react. + With Na in the glass column + Ion exchange occurs, forming radial K + Concentration gradient; (7) After the ion exchange is completed, the temperature is slowly lowered to room temperature. Then the sample is taken out, cleaned and dried, and then subjected to stress relaxation heat treatment: the temperature is raised to 150-240℃ and held for 10-60 hours, and then slowly cooled to room temperature. (8) Cut off both ends of the sample along the axial direction to obtain a preform, and then grind and polish the preform to obtain the infrared radial gradient refractive index lens.

3. The method for fabricating an infrared radial GRIN lens based on chalcogenide glass as described in claim 1, as claimed in claim 2, is characterized in that... In step (1), the raw materials include the corresponding compounds or elemental raw materials used to form GeS2, In2S3 and NaI.

4. The method for preparing an infrared radial GRIN lens based on chalcogenide glass according to claim 1, as described in claim 3, is characterized in that... In step (1), the raw materials are germanium, indium, sulfur and sodium iodide, wherein the purity of germanium, indium and sulfur raw materials is 5N and the purity of sodium iodide raw material is 2N.

5. The method for fabricating an infrared radial GRIN lens based on chalcogenide glass as described in claim 1, according to claim 2, characterized in that, In step (6), the sealed quartz tube is slowly heated to 250-270°C over 2-5 hours and kept at that temperature for 30-40 hours.

6. A method for fabricating an infrared radial GRIN lens based on chalcogenide glass as described in claim 1, as claimed in claim 2, characterized in that, In step (7), after the ion exchange is completed, the process of slowly cooling down to room temperature is as follows: the temperature is slowly cooled down from 250 to 270°C to room temperature within 30 to 50 hours; the specific process of stress relaxation heat treatment is as follows: the temperature is raised from room temperature to 150 to 240°C within 30 to 50 hours and held for 10 to 60 hours, and then slowly cooled down to room temperature within 30 to 50 hours.

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